In the paper and textile industries starch is widely used as a surface size for modifying the properties of yarns, fabrics and papers, and also as a binder for pigments in coated papers and filled textile fabrics. To be effective in such applications the starch must first be hydrated or gelatinized as by heating a water slurry or suspension. If unmodified or native starch is used, the resulting paste viscosity is frequently too high to permit the preparation of a paste of the desired concentration so some sort of starch depolymerization treatment is necessary to enable the desired concentration-viscosity relationship to be obtained. Starch depolymerizing enzymes, such as alpha amylase, are commonly used for this purpose.
When a slurry or water suspension of starch granules is heated, the granules start to swell at the so-called gelatinization temperature and progressively swell further as the temperature is increased. Increases in both the number of swollen granules and in their individual size causes an increase in the viscosity of the paste so that it increases rapidly as cooking proceeds, at least in the initial stages of the cooking process. However, the swollen granules are fragile and tend to fragment under the influence of shear in the cooking vessel so that their individual contribution to the viscosity of the paste is lessened. As a consequence of these two effects, the paste viscosity increases rapidly initially and then at a slower rate as the effect of fragmentation becomes apparent. Where the increase due to additional granule swelling is just balanced by the viscosity lowering due to granule fragmentation, a peak viscosity is obtained. Beyond this point, the effect of the fragmentation becomes predominant and on continued cooking the paste viscosity decreases and eventually stabilizes when both additional swelling and fragmentation become negligible.
In the conventional enzyme depolymerization, the enzyme, usually a bacterial alpha-amylase, is added to the starch slurry along with enzyme stabilizers and pH adjusting agents and the batch is put through a temperature cycle during which the starch is simultaneously gelatinized and depolymerized. When the desired conversion is attained, the temperature of the paste is raised to a level near the boiling point to complete the hydration of the starch and to inactivate the enzyme. While the viscosity vs. time curve for such a conversion varies somewhat from that of a cook of the starch alone, it normally evidences a considerable peak viscosity followed by a viscosity drop similar to that of the starch cook. Especially at higher starch concentrations, such enzyme conversions frequently exhibit high peak viscosities which may limit the concentration of the pastes which can be prepared or which may make necessary the use of heavy and expensive mixing equipment with an accompanying high power input. A variety of systems have been proposed for continuously cooking and enzyme-converting a stream of starch but here also the peak viscosity evidenced by the paste has been troublesome (and often limiting) and has made necessary the use of heavy agitators, scrapers, pumps, etc.
Heretofore, it has been believed that to be effective as a size or adhesive, the starch granules contained therein should all be hydrated and converted to the same degree; and that it was, therefore, necessary that they all be subjected to the same cooking and converting conditions (time-temperature cycle). This, of course, is automatically accomplished in an agitated batch-type conversion or cook. (See, for example, U.S. Pat. No. 3,450,549.)
In continuous conversion systems, such a uniform treatment for each granule is obtained or approached by the use of a relatively long, narrow cooking or hold chamber where the resulting so-called "plug flow" therein results in a reasonably uniform exposure of all of the starch granules to the same conditions as they pass through the chamber. In such a device, the high peak viscosity attained by the paste in the early stages of treatment may make necessary the use of heavy agitators, scrapers, etc., to minimize channelling and dead spots as the starch proceeds through the chamber.
Starch conversion using a plug-flow type reactor column is described in U.S. Pat. No. 3,371,018. A vertical reactor column was equipped with horizontal baffles and with a rotating shaft in the center. As described in the patent, the action was as follows: "The shaft is loaded with flow-directing blades and turbines which impart an internal flow pattern in the column so that the mass has a forward turbulent flow with the minimum of back-mixing. A positive displacement pump forces the slurry into the column and gives the forward flow." Paper mill tests using similar equipment were described in a paper presented by Cave and Adams at a Tappi coating conference in 1968. The mixture of water, starch, enzymes, was pumped into the top of a vertical reactor having a mixer for uniformly applying the feed across a distributor plate. No agitation was provided in the long central section of the reactor where a completely plug-type flow was obtained. From these and similar prior art disclosures, it was known, therefore, that plug-type flow continuous reactors produced enzyme converted starch pastes suitable for use as paper sizes or coatings.
Another procedure which contributes to the uniform exposure of each granule to the same conditions is the use of multiple conversion tanks arranged in series where a step-wise level of conversion is obtained as the product flows from tank to tank. In this instance, also, the peak paste viscosity in the early stages of conversion frequently requires the use of heavy agitation equipment, etc., or limits the concentrations which can be effectively handled.